In operation of a polymer electrolyte fuel cell, an oxygen-containing gas is fed to the cathode and a fuel-containing gas (e.g. hydrogen or methanol) to the anode. Hydrogen (or methanol) in the anode feed gas is then electrochemically oxidised by oxygen, forming water (and carbon dioxide) and generating electricity.
The currently well developed technology of polymer electrolyte membrane (PEM) fuel cells is based on perfluorosulfonic acid polymer membranes (e.g. Nafion®) as electrolyte. The conductivity of this polymer membrane is dependent on the presence of water to solvate the protons from the sulfonic acid groups. Consequently the operational temperature is limited to be below 100° C., typically 60-95° C., at atmospheric pressure. At higher temperatures, the conductivity is reduced dramatically since water is lost. By means of a pressurised system, the operational temperature can be extended but at the expense of overall system efficiency, size and weight. For an operation around 200° C., however, the pressure required will be too high to be of any practical use.
This is unfortunate because the use of a polymer electrolyte membrane at temperatures higher than 100° C. is desirable in several ways. The electrode kinetics will be enhanced and the catalytic activity will be increased at higher temperatures for both electrodes. Another benefit is the reduced poisoning effect of the catalysts by fuel impurities e.g. carbon monoxide, which have been known to be very temperature-dependent, since CO adsorption is less pronounced with increasing temperature. At 80° C., the typical operational temperature of a Nafion®-based polymer membrane electrolyte fuel cell, for example, the CO content as low as 20-50 ppm in the fuel steam will result in a significant loss in the cell performance. As a consequence very pure hydrogen is needed for operation of polymer electrolyte fuel cells.
For applications as a power system for automobiles, the direct usage of pure hydrogen eliminates the need to develop reliable on-board chemical processors; however, it faces other hurdles such as compact and light-weight fuel storage and network for fuel supply and distribution. Instead of pure hydrogen, liquid fuels such as methanol and gasoline/naphtha are the most favourable fuel for automobile applications. Methanol, among others, is for the time being produced in large quantities and is more easily reformed. Although the infrastructure for supply of methanol to the car fleet needs to be evaluated, it is believed that the development of a methanol infrastructure can be more easily obtained than a hydrogen infrastructure.
On-board processing of these high energy density fuels is also an attractive option to attain high vehicle range and short refueling time. For this purpose an on-board fuel processing system is necessary in order to convert the fuel into free hydrogen. During the on-board steam reforming carbon in the fuel is converted into carbon monoxide by oxidation with oxygen from the supplied steam, and hydrogen both from the fuel and from the steam is released as free hydrogen. The reformate gas contains therefore hydrogen, carbon dioxide, carbon monoxide, and the residual water as well as methanol. Due to the above-mentioned CO poisoning effect, further purification of the reformate gas is necessary in order to remove CO down to 10 ppm level. This is carried out by means of a water-gas shift reactor, followed by a preferential oxidiser and/or a membrane separator.
For a small dynamic load as in a vehicle, the main challenge for the on-board processing system is the complexity, which not only requires increased cost, size and volume, but also reduces the start-up time and transient response capacity of the system. Such a fuel processing system generally covers 40-50% cost of the fuel cell power stack. This can be decisively simplified by introducing a CO tolerant polymer electrolyte membrane fuel cell. Direct usage of methanol will be the ultimate option, since the dispensation with the complicated gas processors for reforming and CO removal is very much desired especially for automobile applications. However the technology is far from satisfactory. One of the major challenges is the anodic catalyst. Although Pt/Ru alloy is still recognised as the best, it is not sufficiently active, resulting in high anodic overpotential loss (ca. 350 mV compared to ca. 50 mV for hydrogen) and requiring high catalyst loading of the electrode (3-8 mg/cm2). The insufficient activity of the anode catalyst is due to the slow kinetics of methanol oxidation and the strong poisoning effect of the intermediate species (CO) from methanol oxidation, both expected to be considerably improved by increasing the operational temperature of direct methanol fuel cells (DMFC).
The newest technology in the field is based on potybenzimidazoles (PBI, Celazole® from Hoechst Celanese). U.S. Pat. No. 5,091,087, for example, discloses a process for preparing a microporous PBI membrane. Being sulfonated (see U.S. Pat. No. 4,814,399), phosphonated (see U.S. Pat. No. 5,599,639) or doped with a strong acid (see U.S. Pat. No. 5,525,436 and Journal of Electrochemical society Vol.142 (1995), L21-L23), the polymer membrane becomes a proton conductor at temperatures up to 200° C. This polymer membrane can be used as electrolyte for PEM fuel cells with various types of fuels such as hydrogen, methanol, trimethoxymethane, and formic acid. U.S. Pat. 5,716,727 discloses another method for casting the polymer electrolyte membranes directly from an acid solution. It has been shown that this polymer electrolyte membrane exhibits high electrical conductivity (Journal of Electrochemical society Vol.142 (1995), L21-L23), low methanol crossover rate (Journal of Electrochemical Society, Vol.143 (1996), 1233-1239), excellent thermal stability (Journal of Electrochemical Society, Vol.143(1996), 1225-1232), nearly zero water drag coefficient (Journal of Electrochemical Society, Vol.143(1996), 1260-1263), and enhanced activity for oxygen reduction (Journal of Electrochemical Society, Vol.144(1997), 2973-2982).
It has been suggested that this polymer membrane be used as electrolyte for fuel cells with various types of fuel such as hydrogen (Electrochemical Acta, Vol.41 (1996), 193-197), methanol (Journal of Applied Electrochemistry Vol.26(1996), 751-756), trimethoxymethane (Electrochimica Acta Vol.43(1998), 3821-3828), and formic acid (Journal of Electrochemical Society, Vol.143 (1996), L158-L160). Besides these, this polymer electrolyte membrane has also been used for hydrogen sensors (Journal of Electrochemical Society, Vol.144 (1997), L95-L97), electrochemical capacitor and other electrochemical cells (see for example U.S. Pat. No. 5,688,613).
In addition to the acid-doped polybenzimidazole membrane electrolyte, high performance gas diffusion electrodes are also key components for high temperature polymer electrolyte membrane fuel cells. However, little effort has been made so far in this area, compared with other types of fuel cells such as phosphoric acid fuel cells or Nafion®-based polymer electrolyte membrane fuel cells. In the above-mentioned patents relating to the acid-doped PBI electrolyte fuel cells, little information about the manufacturing of gas diffusion electrodes has been included. There are some indications that phosphoric acid fuel cell electrodes have been used. For example, Wang et al. utilise phosphoric acid fuel cell electrodes, produced by E-TEK (Electrochimica. Acta, vol.41, (1996), 193-197), further treated by impregnation with the polymers. The authors have also made their own electrodes from platinum black, with a very high loading of platinum catalyst (2 mg/cm2). In another publication, Wang et al. use platinum black (Johnson Matthey) and platinum-ruthenium alloy (Giner Inc.) for manufacturing cathode and anode by a filtering-pressing method, also with a very high loading of noble metal catalysts (4 mg/cm2)(Journal of Applied Electrochemistry Vol.26(1996), 751-756).
U.S. Pat. No. 5,599,639 discloses an electrolytic membrane for use in fuel cells. The fuel cell assembly comprises a carbon paper substrate, a catalyst layer using a fluorinated resin as a binder, and polybenzimidazole resin electrolytic membrane. No specific information about the performance of such membranes in fuel cells was given.
U.S. Pat. No. 4,647,359 discloses a catalytic sandwich comprising a open pore carbon cloth and having on the one side a layer of catalytic carbon, i.e. platinum/carbon and a hydrophobic binder (e.g. PTFE), and on the other side a layer of a non-catalytic carbon in a hydrophobic binder (e.g. PTFE). The sandwich may be prepared by press fitting.
J. Electrochem. Soc. Vol. 143 (1996), L158-160, describe a fuel cell for oxidation of formic acid. The fuel cell comprises a polymer electrolyte comprising a phosphoric acid doped (5 moles H3PO4 per repeat unit) polybenzimidazole (PBI) as polymer electrolyte and electrodes including platinum black and platinum-ruthenium as catalyst, respectively. The catalyst loading was 4 mg/cm2 and the electrodes were impregnated with 15-21 μl/cm2 of 5 M H3PO4. The membranes were assembled by hot-pressing.
WO 99/04445 describes a membrane electrode constructed by providing successive layer of an electrode, a PBI paste/gel, a polymer fabric, a PBI paste/gel, and an electrode.